Ruthenium-catalyzed propargylic reduction of propargylic alcohols with silanes

Article abstract of DOI:10.1002/anie.200601181

Ru₂ can do it! Substitution of the OH moiety in propargylic alcohols by hydride proceeds smoothly with triethylsilane by catalysis with the thiolate-bridged diruthenium complex 1 (see scheme; Cp* = η⁵-C₅Me₅). Th

Full text of DOI:10.1002/anie.200601181

Angewandte
Chemie
Synthetic Methods
DOI: 10.1002/anie.200601181
Ruthenium-Catalyzed Propargylic Reduction of
Propargylic Alcohols with Silanes**
Yoshiaki Nishibayashi,* Akira Shinoda,
Yoshihiro Miyake, Hiroshi Matsuzawa, and
Mitsunobu Sato
Dedicated to Professor Sakae Uemura
on the occasion of his 65th birthday
A number of transition-metal complexes are known to
promote the hydrosilylation of alkynes with silanes to give
the corresponding vinyl silanes, which are versatile building
blocks in organic synthesis.[1] The development of highly
stereo- and regioselective hydrosilylation reactions of alkynes
with silanes is an important subject in organic synthesis.[1] The
selectivity in transition-metal-catalyzed hydrosilylation
depends on the nature of catalysts, substrates, silanes, and
solvents.[2,3] Recently, Trost and co-workers reported a highly
selective preparation of a-vinyl silanes from a wide range of
[*] Prof. Dr. Y. Nishibayashi, Dr. Y. Miyake, Dr. H. Matsuzawa
Institute of Engineering Innovation
The University of Tokyo
Yayoi, Bunkyo-ku, Tokyo, 113-8656 (Japan)
Fax: (+81)3-5841-1175
E-mail: ynishiba@sogo.t.u-tokyo.ac.jp
A. Shinoda, Prof. Dr. M. Sato
Department of Material Science and Technology
Faculty of Engineering
Kogakuin University
Hachioji, Tokyo, 192-0015 (Japan)
[**] This work was supported by a Grant-in-Aid for Scientific Research
for Young Scientists (A, No. 15685006) and Grant-in-Aid for
Scientific Research on Priority Areas (No. 18037012) to Y.N. from
the Ministry of Education, Culture, Sports, Science and Technology,
Japan.
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alkynes by catalysis with the cationic ruthenium complex
[Cp*Ru(MeCN)3]PF6 (Cp* = h5-C5Me5).[2a,d,f]
We have recently disclosed a novel catalytic activity of
thiolate-bridged diruthenium complexes[4]
such
[{Cp*RuCl(m2-SR)}2] (R = Me (1a), nPr, iPr (1b)) and
[Cp*RuCl(m2-SMe)2RuCp*(OH2)]OTf (1c; OTf =
as
OSO2CF3). These complexes were shown to be suitable
catalysts for the propargylic substitution reactions of prop-
argylic alcohols with a variety of heteroatom- and carbon-
centered nucleophiles to give the corresponding propargylic-
substituted compounds in high yields with complete selectiv-
ity.[5] Some other groups reported the transition-metal-
catalyzed propargylic allylation of propargylic alcohols with
allyltrimethylsilane to give the corresponding allylated prod-
ucts in good to high yields with high selectivity.[6] These results
first prompted us to investigate the ruthenium-catalyzed
allylation of propargylic alcohols with allylsilane. As a result,
we comfirmed that only cationic thiolate-bridged diruthe-
nium complexes promote the substitution by an allyl group of
the hydroxy moiety of propargylic alcohols that bear an
internal alkyne, as was expected (Scheme 1).
Scheme 2. Diverging pathways in ruthenium-catalyzed transformations
of propargylic alcohols with triethylsilane.
of our knowledge. Preliminary results of the ruthenium-
catalyzed propargylic reduction of propargylic alcohols with
triethylsilane are presented herein. It is noteworthy that the
reactions of propargylic alcohols with pinacolborane in the
presence of a catalytic amount of 1a unexpectedly gave the
reductive homocoupling products of the propargylic alcohols
in good yields with high selectivity.[11]
Treatment of 1-(4-methylphenyl)-3-phenyl-2-propyn-1-ol
(2a) with triethylsilane in the presence of the cationic
diruthenium complex 1c (5 mol%) in ClCH2CH2Cl at 808C
for one hour afforded the propargylic reduction product 1-
phenyl-3-(4-methylphenyl)-1-propyne (3a) in 77% yield after
isolation (Table 1, entry 1). The reaction also proceeded quite
Table 1: Propargylic reduction of propargylic alcohol 2a with various
silanes in the presence of 1c.[a]
Entry
Silane (equiv)
t [h]
Yield of 3a [%][b]
77
59
67
50
15
0
1
2
3
4
5
Et3SiH(10)
Et3SiH(5)
tBuMe2SiH(10)
PhMe2SiH(10)
iPr3SiH(10)
1
1
1
1
3
3
Scheme 1. Ruthenium-catalyzed propargylic substitution reactions of
6
Ph3SiH(10)
propargylic alcohols with allyltrimethylsilane.
7
8
9
10
Ph2SiH2 (10)
PhMeSiH2 (10)
PhSiH3 (10)
[MeSi(H)O]n (10)[c]
1
0
1
1
1
0
0
0
As an extension of our study on the catalytic reactions
with other organosilicon compounds, we have now observed
the unexpected and completely chemoselective reduction at
the propargylic position of propargylic alcohols that bear an
internal alkyne with a variety of silanes to give the corre-
sponding reduction products (substitution of the OH moiety
by hydride, referred to herein as propargylic reduction) in
good to high yields. This result is in sharp contrast to the
recently reported selective formation of (E)-vinyl silanes by
the hydrosilylation of propargylic alcohols with silanes which
was catalyzed by the cationic ruthenium complex [Cp*Ru-
(MeCN)3]PF6 (Scheme 2).[2c,e] Although some transition-
metal complexes other than ruthenium are also known to
promote the propargylic substitution reactions of propargylic
alcohol derivatives with a variety of heteroatom- and carbon-
centered nucleophiles to afford the corresponding function-
alized propargylic compounds,[6–10] the catalytic propargylic
reduction with a hydride has not yet been reported to the best
[a] All reactions of 2a (0.30 mmol) with silane were carried out at 808C in
the presence of 1c (0.015 mmol) in ClCH2CH2Cl (10 mL). [b] Yield of
isolated product. [c] Poly(methylhydrosiloxane) was used.
smoothly at room temperature with a slight decrease of the
product yield. This reductive reaction did not proceed at all
when either neutral thiolate-bridged diruthenium complexes
1a and 1b or mono- and polynuclear ruthenium complexes
such as [(h5-C9H7)RuCl(PPh3)2], [{Cp*RuCl2}2], and
[{Cp*RuCl}4], were employed as catalysts. Although gold-
catalyzed propargylic substitution reactions of propargylic
alcohols with nucleophiles were reported by Campagne and
co-workers,[6b] the present reductive reaction hardly pro-
ceeded when AuCl3 was used as a catalyst in place of 1c under
similar conditions. Other silanes such as tert-butyldimethylsi-
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lane, dimethylphenylsilane, and triisopropylsilane also
worked as reducing agents (Table 1, entries 3–5), but triphe-
nylsilane, diphenylsilane, methylphenylsilane, phenylsilane,
and poly(methylhydrosiloxane) were not effective (Table 1,
entries 6–10).
Reactions of a variety of propargylic alcohols 2 with
triethylsilane were subsequently investigated. Typical results
are shown in Table 2. The presence of an electron-donating
Table 2: Propargylic reduction of propargylic alcohols 2 with triethylsi-
lane in the presence of 1c.[a]
Scheme 3. Ruthenium-catalyzed reactions of propargylic alcohols with
triethylsilane.
Entry
2
t [h]
Yield of 3 [%][b]
R
R’
1
2
3
4
5
6
7
8
9
p-MeOC6H4
p-MeC6H4
Ph
p-ClC6H4
p-FC6H4
p-CF3C6H4
p-MeOC6H4
p-MeOC6H4
p-MeOC6H4
1-naphthyl
2-naphthyl
Ph
Ph
Ph
Ph
Ph
Ph
nBu
tBu
SiMe3
Ph
(2b)
(2a)
(2c)
(2d)
(2e)
(2 f)
(2g)
(2h)
(2i)
1
1
1
5
5
23
1
1
1
3
97
77
43
42
46
0
94
93
99
66
55
51[c]
phenyl)-1-propyne ([D1]3b) was produced with a high
deuterium incorporation at the propargylic position
(95% D) [Eq. (1)]. This result indicates that triethylsilane
10
11
12
(2j)
(2k)
2l)
Ph
3
1
=
Ph2C CHPh
(
works as a reduing agent for the reduction at the propargylic
position. The pronounced electronic effect of the substituent
at the para position of the benzene ring in the propargylic
alcohol substrate (see above) leads us to propose the presence
of a propargylic cation as a reactive intermediate, in which the
alkyne moiety may interact with the ruthenium centers of the
thiolate-bridged diruthenium complex, although direct evi-
dence of such a reactive intermediate has not yet been
obtained. A nucleophilic attack of hy-
dride from the triflate-activated silane
on this ruthenium-coordinated propar-
gylic cation might give the correspond-
ing reduction product. A proposed reac-
[a] All reactions of 2 (0.30 mmol) with triethylsilane (3.00 mmol) were
carried out at 808C in the presence of 1c (0.015 mmol) in ClCH2CH2Cl
(10 mL). [b] Yield of isolated product. [c] 10 mol% of 1c was used.
group such as methoxy and methyl on the propargylic
benzene ring (2b and 2a) increased the yield of the
propargylic reduction product (Table 2, entries 1 and 2;
compare with entry 3). In contrast, no improvement of the
yield of the reduced products was observed when an electron-
withdrawing group such as chloro or fluoro was introduced
onto the propargylic benzene ring (2d and 2e; Table 2,
entries 4 and 5), and no reduction proceeded at 808C even for
23 hours when a trifluoromethyl moiety (2 f) was introduced
(Table 2, entry 6). These results suggest that propargylic
cations might be involved as reactive intermediates. In the
case of reactions of propargylic alcohols that bear a naphthyl
moiety (2j and 2k), the corresponding reduced products were
formed in good yields (Table 2, entries 10 and 11). From an
alkenyl-substituted propargylic alcohol (2l), the correspond-
ing 1,4-enyne compound was obtained in moderate yield
(Table 2, entry 12). However, when 1,1,3-triphenyl-2-propyn-
1-ol (2m), 2-methyl-4-phenyl-3-butyn-2-ol (2n), 2,4-diphenyl-
3-butyn-2-ol (2o), 1-cyclohexyl-3-phenyl-2-propyn-1-ol (2p),
1-phenyl-2-propyn-1-ol (2q), and 3-phenyl-2-propyn-1-ol (2r)
were used as substrates, the propargylic reduction did not
proceed under the same reaction conditions, and some
unexpected reactions occurred (Scheme 3).[12]
tive intermediate in the propargylic
reduction is shown in Scheme 4. This
proposal may be supported by the
observation that no reduction occurred
in the reactions of allylic alcohols and
secondary alcohols such as cinnamyl
alcohol and phenylethyl alcohol with
triethylsilane under the same reaction
conditions.
Scheme 4. A pro-
posed reactive inter-
mediate.
Nicholas and Siegel have reported the propargylic reduc-
tion of [Co2(CO)6]-complexed propargylic cations, which
were prepared by reactions of propargylic alcohols with a
stoichiometric amount of [Co2(CO)8], with sodium borohy-
dride.[13] In this stoichiometric reaction, several steps were
necessary to obtain the corresponding reduction products.
However, catalytic and direct reduction of secondary or
tertiary alcohols and primary alcohols with silanes to give the
corresponding alkanes in high to excellent yields has been
reported by Baba and co-workers[14] and by Yamamoto and
To elucidate the mechanism of the propargylic reduction,
the following reactions were investigated. When 2b was
treated with [D1]triethylsilane (97% D) in the presence of a
catalytic amount of 1c, deuterated 1-phenyl-3-(4-methoxy-
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co-workers[15] by using indium trichloride and tris(pentafluoro-
and 2a (66.7 mg, 0.30 mmol) and triethylsilane (0.48 mL, 3.00 mmol)
were added. The flask was kept at 808C for 1 h with magnetic stirring.
After the flask had cooled, the solvent was concentrated in vacuo, and
the residue was purified by column chromatography on SiO2 with
EtOAc/n-hexane (5:95) to give 3a[16] as a yellow oil (47.6 mg,
0.231 mmol, 77% yield). 1H NMR: d = 2.33 (s, 3H), 3.78 (s, 2H),
7.12–7.44 ppm (m, 9H); 13C NMR: d = 21.0, 25.3, 82.4, 87.8, 123.7,
127.7, 127.8, 128.2, 129.2, 131.6, 133.7, 136.1 ppm.
phenyl)borane, respectively, as catalysts. During our work,
the propargylic reduction of propargylic acetates with trie-
thylsilane by use of a catalytic amount of indium tribromide[16]
was reported, in which the propargylic reduction of prop-
argylic alcohols did not proceed smoothly. The reaction
described herein provides the first example of the catalytic
and direct propargylic reduction of propargylic alcohols to
form the corresponding reduced products in good to high
yields, although the scope of the reaction is still limited.
Furthermore, we have investigated the ruthenium-cata-
lyzed reactions of propargylic alcohols with organosilicon
compounds other than trialkylsilanes under the same reaction
conditions. Interestingly, the reactions of 2a with allyltrime-
thoxysilane and triethoxysilane at 808C for one hour afforded
the corresponding ethers 6a and 6b in 81% and 88% yields,
respectively, after isolation (Scheme 5). The reaction of 2a
3l: Brown oil. Yield: 51%. 1H NMR: d = 3.22 (d, J = 7.3 Hz, 2H),
6.18 (t, J = 7.3 Hz, 1H), 7.29–7.36 ppm (m, 15H); 13C NMR: d = 20.6,
80.9, 88.1, 123.4, 127.3, 127.4, 127.7, 128.1, 128.2, 128.4, 128.6, 128.7,
129.8, 131.6, 139.2, 142.0, 143.3 ppm. HRMS: calcd for C23H18
294.1409; found: 294.1404.
:
Received: March 24, 2006
Revised: April 30, 2006
Published online: June 27, 2006
Keywords: homogeneous catalysis · reduction · ruthenium ·
.
silanes · synthetic methods
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Scheme 5. Ruthenium-catalyzed propargylic substitution reactions of
propargylic alcohol with organosilicon compounds.
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with allyldimethylsilane under the same reaction conditions
led to the formation of the allylated compound 7a in 62%
yield after isolation (Scheme 5).[17] These results indicate that
the ability of substituent transfer from a silicon center to the
propargylic position has the following trend in these catalytic
reactions: alkoxy > allyl > hydride @ alkyl, aryl.
In summary, we have observed novel ruthenium-catalyzed
propargylic reduction of propargylic alcohols with triethylsi-
lane to give the corresponding alkynes in good to high yields
with complete selectivity. The transition-metal-catalyzed
propargylic reduction of propargylic alcohols has until now
been an unknown reaction system, in contrast to the recently
reported transition-metal-catalyzed propargylic substitution
reactions of propargylic alcohol derivatives with nucleophiles.
Further investigations for elucidating the reaction mechanism
in detail and for broadening the synthetic application of this
propargylic reduction are currently in progress.
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Experimental Section
Typical procedure for the propargylic reduction of propargylic
alcohol (2a) with triethylsilane, catalyzed by 1c: Compound 1c
(11.5 mg, 0.015 mmol) was placed in a 20-mL flask under N2. Distilled
and degassed 1,2-dichloroethane (10 mL) was then added to the flask,
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4838
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[17] Separately, we investigated the reaction of 2a with a mixture of
allyltrimethylsilane and triethylsilane in the presence of 5 mol%
of 1c at 808C for 1 h; only 7a was isolated in 81% yield.
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